The effect of diet on colon carcinogenesis was studied by assaying enzymes in the intestinal microflora that are known to form proximal carcinogens. Rats were placed on a grain diet and then shifted to a beef diet; their fecal specimens were assayed for beta-glucuronidase, nitroreductase, azoreductase and beta-glucosidase. Rats on the grain diet maintained a constant level of specific activities for the four fecal enzymes for a 30-40 day period. After changing to the high beef diet, the specific activities of beta-glucuronidase, azoreductase and nitroreductase increased between 1.5 and 2.5 fold within two weeks and then remained constant. (The specific activity of beta-glucosidase decreased after the dietary shift, probably due to the lack of beta-glucosidic bonds in the experimental diet). The results indicate that a meat diet influences the bacterial enzyme levels in the large bowel. The increase of beta-glucuronidase, azoreductase and nitroreductase leads to the reduction of carcinogens from certain chemical inducers and food additives in experimental animals. These studies may provide insight to the high incidence of colon cancer in people eating a high meat, Western-type diet.